Alloys which are usable at elevated temperatures find widespread application, particularly the gas turbine field, and other fields including furnaces and other thermal processing equipment. Most alloys used in gas turbine engines are based on nickel. Nickel alloys, usually referred to as superalloys, have useful strengths of up to about 2200.degree. F. Nickel base superalloys are now being used very near their melting points and significant increases in use temperature will undoubtedly require the adoption of different alloy systems.
It is known that the so-called refractory metals, molybdenum, tungsten, tantalum, and niobium have exceptionally high melting points. Of these, niobium has other favorable properties, and considerable efforts were made to develop niobium based alloys in the 1950s and 1960s. These efforts failed because of the oxidation susceptibility of niobium.
In the field of high temperature materials, it is common practice to use protective coatings to provide oxidation resistance to materials which have useful properties, but lack inherent oxidation resistance. In the development of niobium alloys, efforts were made to use refractory silicide coatings, but without significant success. The coatings developed had a coefficient of thermal expansion substantially greater than that of the niobium substrate, and were characterized by significant cracking. Cracking permitted the passage of oxygen through the coating into the substrate, causing early substrate failure. In current, very limited usage of niobium alloys in gas turbine engines, silicide coatings are used, but the problems relating to cracking persist.
In a prior application, U.S. Ser. No. 07/286,835 filed on Dec. 20, 1988, it was proposed to use a two layer coating system to protect niobium and niobium based alloys. This case has been indicated as being allowable and the contents thereof are incorporated herein by reference. The coating system comprised a first coating of Ta.sub.5 Si.sub.3, and/or Nb.sub.5 Si.sub.3 on the substrate with an outer layer comprised essentially of MoSi.sub.2. The theory behind this dual layer coating system was that the first layer provided diffusional stability by minimizing the diffusion of silicon from the MoSi.sub.2 into the substrate, and the MoSi.sub.2 layer provided oxidation resistance. Upon more extensive testing of this coating, it has been found to have a mismatch in coefficient of thermal expansion between the niobium substrate and the coating and is prone to crack when subjected to thermal cycling.